The top of a lake gets frozen at a place where the surrounding air is at a temperature of $- 20^{\circ} C$ . Then choose the correct statement.

The temperature of the layer of water in contact with the lower surface of the ice block wil be at $0^{\circ} C$ and that at the bottom of the lake will be at $4^{\circ} C$ .

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The temperature of water below the lower surface of ice will be $4^{c i r c} C$ right up to the bottom of the lake.

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The temperature of the water below the lower surface of ice will be $0^{\circ} C$ right up to the bottom of the lake.

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The temperature of the layer of water immediately in contact with the lower surface of ice will be $- 20^{\circ} C$ and that of water at the bottom will be $0^{\circ} C$ .

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Solution

The correct option is A
The temperature of the layer of water in contact with the lower surface of the ice block wil be at $0^{\circ} C$ and that at the bottom of the lake will be at $4^{\circ} C$ .

The density of water is maximum at $4^{\circ} C$ and it decreases as the temperature is lowered to $0^{\circ} C$ . Since the water at $4^{\circ} C$ is most dense, it will sink to the bottom of the lake. The water at $0^{\circ} C$ is the least dense, therefore, it will rise to the top of the lake. Hence, the correct choice is (a).

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Similar questions

Q. The top of the lake is frozen. The air in contact with the surface of lake is at

- 15^{o} C

. Then the maximum temperature of the water in contact with the lower surface of ice will be

The top of a lake is frozen when the air in contact with the lake surface is at $- 5^{0} C$ . The temperature of water in contact with the bottom of the lake will be

Q. The temperature of water at the bottom of a lake whose upper surface has frozen to ice would be around ______________.

Water is densest at 4 $^{\circ}$ C; this is the reason lakes do not completely freeze during extreme winters. Say, the atmosphere is at - $θ^{\circ} C (θ > 0)$ ,now the temperature of the lake's water starts to fall, and the denser water from the top sinks, getting the bottom layers up, and cooling them until the temperature reaches 4 $^{\circ}$ C at the upper surface. Now, further reduction in temperature actually makes the water less dense! Hence the colder water stays on top, starts to freeze, until the top layer of ice reaches - $θ^{\circ} C$ and the bottom layer at 0 $^{\circ}$ C . And this layer of ice is the only area through which the water below loses heat (by conduction), slowly increasing the thickness of the ice (because ice is a bad conduction), and for a very long time the water at the bottom of the lake remains at 4 $^{\circ}$ C! Now, Let the thickness of the layer of ice be $y_{1}$ , at an instant.How much time t, will it take to increase to thickness $y_{2}$ ?

$ρ \to$ density of the ice

K $\to$ thermal conductivity of ice.

L $\to$ Latent heat of fusion

$θ \to$ temperature of surface (as mentioned earlier)

$ρ \to$ density of the ice

K $\to$ thermal conductivity of ice.

L $\to$ Latent heat of fusion

$θ \to$ temperature of surface (as mentioned earlier)

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The top of a lake gets frozen at a place where the surrounding air is at a temperature of −20∘C. Then choose the correct statement.

The top of a lake gets frozen at a place where the surrounding air is at a temperature of $- 20^{\circ} C$ . Then choose the correct statement.